1. Field of the Invention
The invention relates generally to a lighting arrangement having light sources emitting light at different wavelengths, and means for adjusting the relative light outputs of the different light sources.
2. Description of the Related Art
Artificial lighting, in particular in outdoor settings, operates at luminance levels associated with mesopic vision. There is a growing recognition that for mesopic conditions the output efficiency of a lighting arrangement expressed in Lumen/Watt, is a poor measure of a light arrangement's performance and perception and that the scotopic/photopic ratio of the emitted light must be taken into account.
US 2012/0169239 A1 discloses a lighting arrangement comprising a light sensor and multiple light emitting units. The light emitting units can be independently controlled in order to generate light having an optimum S/P ratio for the prevailing luminance conditions. The lighting arrangement is capable of emitting light at 555 nm, so that light having low S/P ratio can be emitted under photopic conditions.
EP 2 246 611 A1 discloses bi-chromatic light sources with two different wavelengths, a first wavelength in the range from 560-610 nm and a second wavelength in a range of either 500-550 nm or 610-660 nm. The light source of the first wavelength emits light continuously. The light source of the second wavelength is switched on during critical periods. The intensities of the two light sources are not modulated.
US 2008/0252197 A1 discloses a white light source with tunable color temperature. The light source contains two light emitting units, which can be phosphor conversion LEDs. The two light emitting units comprise phosphors emitting light of different color temperatures. The color temperature of the light source can be adjusted by adjusting the power sent to each of the two light emitting units.
Optimizing the spectral distribution for a high S/P ratio by itself does not take into account the importance of optimizing the spectral distribution for contrast and/or color vision. Prior art lighting arrangements also fail to take into account that blue light, although favorable for a high S/P ratio, is poorly processed by the eyes of elderly people due to yellowing of eye lenses with age. As yellow filters out blue light, up to 50% or more of the blue part of the spectrum does not reach the retina of the eye of an older person.
Thus, there is a need for a lighting arrangement permitting adjustment of the spectral distribution in response to luminance level, to optimize contrast and/or color vision while maintaining a high S/P ratio. There is a further need of such lighting arrangement that avoids the use of a wideband V(λ) phosphor conversion LED having a maximized luminous emission at 555 nm.